One of the primary purposes of the crankcase oil is to prevent engine wear. The wear requirements for crankcase oils will become more stringent during the 1990's. Gasoline engine manufacturers are requesting lower phosphorus oils to prevent catalyst and oxygen sensor deactivation, while still maintaining minimum valve train wear.
One obvious approach is to take advantage of the powerful wear inhibiting properties of the zinc dialkyl dithiophosphates (ZnDTP) that constitute the major wear inhibiting additives in modern crankcase formulations. In particular, zinc dialkyl dithiophosphates can be primary alcohol derived or secondary alcohol derived. In recent studies, S. H. Roby of Lubrizol, reported that secondary alcohol-derived ZnDTP appears to moderate abrasion, fatigue, and pitting wear rates. See "Investigation of Sequence IIIE Valve Train Wear Mechanism," Lubr. Eng., 47, 5, pp 413-422 (1991). The primary alcohol-derived ZnDTP formulation was not effective in reducing valve train wear in full-length 64 hour Sequence IIIE engine tests. These studies were done at current phosphorus levels of 0.11-0.13%.
One possible approach would be to maximize the amount of secondary alcohol-derived ZnDTP in the formulation for effective wear protection at low phosphorus levels. It is conceivable, however, that this approach might someday become impractical, when the wear inhibiting properties of the secondary alcohol-derived ZnDTPs become ineffective at increasingly lower phosphorus levels.
Another, equally plausible approach, is to boost the valve train wear performance of the primary alcohol-derived ZnDTP with ashless, nonphosphorus-containing inhibitors. In this approach, one adds a new inhibitor to a baseline formulation containing primary alcohol-derived ZnDTP. The combined effect should turn a marginal wear inhibiting package into a package with good valve train wear protection. In addition, the relative thermal stability of the primary ZnDTP versus the secondary ZnDTP maintains strong oxidation inhibition characteristics. Such cost/performance properties are a prime consideration for formulating oils.
It is well known that hydrocarbon oils are partially oxidized when contacted with oxygen at elevated temperatures for long periods. The internal combustion engine is a model oxidator, since it contacts a hydrocarbon motor oil with air under agitation at high temperatures. Also, many of the metals (iron, copper, lead, nickel, etc.) used in the manufacture of the engine and in contact with both the oil and air, are effective oxidation catalysts, which increase the rate of oxidation. The oxidation in motor oils is particularly acute in the modern internal combustion engine that is designed to operate under heavy work loads and at elevated temperatures.
The oxidation process produces acidic bodies within the motor oil that are corrosive to typical copper and lead engine bearings. It has also been discovered that the oxidation products contribute to piston ring sticking, the formation of sludge within the motor oil, and an overall breakdown of viscosity characteristics of the lubricant.
Several effective oxidation inhibitors have been developed and are used in almost all of the conventional motor oils today. Typical of these inhibitors are the sulfurized oil-soluble organic compounds, such as aromatic or alkyl sulfides and polysulfides, sulfurized olefins, sulfurized carboxylic acid esters, and sulfurized ester-olefins, as well as the oil-soluble phenolic and aromatic amine antioxidants. These inhibitors, while exhibiting good antioxidant properties, are burdened by economic and oil contamination problems.
It is therefore natural that modern formulations rely on zinc dithiophosphates to improve their load-bearing properties, such as extreme pressure and antiwear properties, and oxidation inhibition.
ZnDTP is one of the metal salts of dihydrocarbyl dithiophosphoric acids. Metal salts of dihydrocarbyl dithiophosphoric acids are well known as load-bearing additives for lubricating oils. Such salts may be represented by the formula: ##STR1## wherein: R is the same or different optionally substituted hydrocarbyl group;
M is a metal, and PA1 n corresponds to the valence of the metal M.
Many types of metal salts of dihydrocarbyl dithiophosphoric acids have been proposed. U.S. Pat. Nos. 2,410,642, 2,540,084, and 4,212,751, and U.K. Patents 723,133 and 852,365 proposed those in which the optionally substituted hydrocarbyl groups represented by R are the same or different alkyl, cycloalkyl, aryl groups. U.S. Pat. Nos. 3,102,096 and 4,288,335, and U.K. Patent 2,070,054 disclose groups derived from alkoxylated alcohols and monoester alcohols.
U.S. Pat. No. 4,466,895 discloses metal salts of one or more dialkyl phosphorodithioic acids where a mixture of primary and secondary alcohols is used as a starting material, and the total number of carbon atoms per phosphorus atoms is less than 8. Preferably, n-butyl and isopropyl alcohols are used in this invention.
Zinc dithiophosphates can be derived from primary alcohols, secondary alcohols, and phenols. Zinc dithiophosphates derived from primary or secondary alcohols have certain advantages over the alkaryl type of zinc dithiophosphate. These advantages include (1) good antiwear properties in 0.10-0.14% P-containing oils, if properly formulated, and (2) relatively cheap oxidation inhibition properties. While primary alcohol-derived zinc dithiophosphates are more thermally stable than secondary alcohol-derived zinc dithiophosphates, primary alcohol-derived zinc dithiophosphates have the disadvantage of not being as effective in valve train wear inhibition as the secondary alcohol-derived zinc dithiophosphates. This could be a real problem in the low phosphorus oil environment ahead.